The present invention relates to a developing device to be used for electrophotographic image formation apparatuses such as printers and copiers.
In electrophotographic image formation apparatuses such as printers and copiers, there has conventionally been used a developing device which develops with toner an electrostatic latent image formed on a surface of a photoconductor to visualize the image. This developing device generally has a developing roller or developing sleeve which rotates while holding charged toner on the outer peripheral surface in a thin layer state, so that toner is fed onto the photoconductor from the developing roller or the like.
Meanwhile, as printers have been wide-spreading in offices rapidly in recent years, users have been becoming increasingly more conscious of image quality and cost. One of image noise is ‘fogging’ which occurs due to deposition of the toner at portions other than the electrostatic latent image on the photoconductor. This occurs more often due to a deterioration of toner charge amount resulting from a deterioration of the toner within the developing device during endurance time. The deterioration of toner charge amount due to endurance causes not only image deteriorations but also increases in toner consumption, which substantially causes larger burdens of cost on users.
For electric charging of the toner in the developing device, there is a method that a blade-like member is brought into press contact against the developing roller and then the toner held on the developing roller in a thin layer state is electrically charged with a DC voltage applied thereto by the blade-like member. However, a nip width formed between the blade-like member and the developing roller, if small, would make it hard to impart a sufficient charge amount to deteriorated toner. Thus, for obtainment of larger nip width between the blade-like member and the developing roller, there have been proposed methods for bringing a flexible film-like member into contact with the developing roller in Japanese Patent Laid-Open Publications No. S63-155065, H05-11583, H05-224517, H09-90744, H10-31358, H10-133474, and H11-272069.
In the case where the nip width is broadened by using a film-like member and a DC voltage is applied to the toner layer via the film-like member, although even toner in a low-chargeability state can be charged up to a proper charge amount (e.g., −15 to −40 μc/g) so that occurrence of fogging can be reduced, yet toner in a good chargeability conversely would be overcharged (e.g., −40 μc/g or more). Overcharged toner, which is strongly restrained onto the developing roller by coulomb force, might become less likely to separate from the developing roller, adversely affecting the developability.
As a countermeasure for such overcharging, it is conceivable to lower the level of the DC voltage. However, such a countermeasure would lead to a weakening of the electric field for restraining the toner onto the developing roller, there would arise another issue that toner would stick to the film-like member.